Probably after the first two lines sung by the singer. I will know straight away if I like the song or not. The chorus is not that important to me. There are many songs whose verses I like, but whose chorus I find weak.

This said, I am generally curious and will listen to the whole song before making a final decision. There are surprises, and some songs really grow on me.

Often, I can be put off by the voice. You can give me the best song in the world, but if it is sung by Joe Elliott, Vince Neil, John Waite or Sebastian Bach for instance, there is no chance I will like it. I won't be able to get over the voice and the delivery. The contrary is true as some singers can turn average songs into good songs.